In recent years, plastic optical fibers have been developed, providing advantageous features: they are inexpensive and lightweight fibers set with a greater diameter, and end-face processing and handling are easier to conduct. Plastic optical fibers have been used in various applications, for example, lighting, sensors, interior wiring such as FA, OA and LANs, and wiring in mobile bodies for short- to medium-distance communication systems such as automobile LANs.
When conventional single-core plastic optical fibers are bent with a smaller flexural radius, greater bending loss is observed. Multicore plastic optical fibers have been proposed to reduce bending loss by employing a structure where multiple cores are individually partitioned using cladding material (cladding). Also, to expand an application environment, there is demand for development of multicore plastic optical fibers with excellent heat resistance.
JP H11-95048A (Patent Publication 1) proposes a multicore plastic optical fiber cable that exhibits excellent heat resistance by using PMMA resin to make multiple cores, by forming cladding with a specifically formulated vinylidene fluoride-tetrafluoroethylene-hexafluoropropene copolymer, and by coating the optical fiber using vinylidene fluoride resin to provide protection.
JP H09-33737A (Patent Publication 2) discloses a method for reducing bending loss by setting a specific range for the difference in refractive indices between the cores and cladding of a multicore plastic optical fiber and for the ratio of cross-sectional areas between the core and cladding.
WO1995/32442 (Patent Publication 3) discloses a method for setting a specific range for the difference in refractive indices between the cores and cladding of a multicore plastic optical fiber so that transmission loss is minimized in a broader bandwidth and bending loss is also reduced.
WO1998/35247 (Patent Publication 4) discloses a technology capable of reducing bending loss and increasing the light receiving capacity by forming the cladding of a multicore plastic optical fiber to be double-layered with different refractive indices.
JP H11-160553A (Patent Publication 5) discloses a technology capable of reducing bending loss and increasing the light receiving capacity of a multicore plastic optical fiber by specifying the size of cross-sectional areas of the cores and by setting the shape of central cores to be hexagonal.
Moreover, to use plastic optical fibers as interior wiring systems of mobile bodies such as automobile LANs or aircraft LANs that require high heat-resistance properties, various materials are proposed for coating the exterior surface of a plastic optical fiber so as to provide heat resistance properties.
JP2006-215178A (Patent Publication 6) discloses a technology capable of enhancing heat resistance by coating plastic optical fibers with resin having polypropylene resin as its main component.
JP2005-266742A (Patent Publication 7) discloses technology capable of enhancing heat resistance by coating plastic optical fibers with water-crosslinked polyethylene.